The present embodiments relate generally to sensors for heat flux differential scanning calorimeters.
Heat flux differential scanning calorimeters (DSCs) generally use a method of measurement based upon a local temperature difference. Heat flux DSCs are twin instruments that measure the difference in heat flow rates between a sample measuring system and a reference measuring system. Most typically, heat flux DSCs measure a single temperature difference between the sample and reference systems.
The measured heat flow rate is taken to be:
      q    =                  Δ        ⁢                                  ⁢        T                    R        ⁡                  (          T          )                      ,
where ΔT is the temperature difference measured between the sample and reference systems and R(T) is a temperature dependent proportionality factor that has the dimensions of thermal resistance, i.e., temperature divided by power. For example, the unit may be of measurement ° C./watt.
The temperature difference may be measured by any convenient technique, for example by using thermocouples. The temperature difference may be measured by a single differential thermocouple where one thermocouple junction is attached to the sample system and a second thermocouple junction is attached to the reference system, and the two junctions are connected as a differential pair. In a differential pair, the positive leads of the two thermocouples are electrically connected and the temperature difference is measured between the negative leads of the thermocouple pair. Alternatively, the negative leads of the two thermocouples can be electrically connected and the temperature difference is measured between the positive leads of the thermocouple pair.
A useful figure of merit for a DSC sensor is the product of the electrical output of the sensor and the thermal resistance of the sensor. It is a measure of the sensitivity of the sensor, the ratio of electrical output per unit of power, e.g. μvolts/watt. For a differential thermocouple, it is the product of the Seebeck coefficient of the thermocouple and the sensor thermal resistance.
One method for increasing the sensitivity of the sensor is to use a thermopile, which is a number of thermocouples in series, to measure the temperature difference. In a thermopile, an equal number of thermocouple junctions are installed on each of the sample and reference systems. The junctions are connected in series with alternate junctions on the sample and reference systems. For example, the positive lead of a sample junction connects to the positive lead of a reference junction and the negative lead of the sample junction connects to the negative lead of another reference junction.
The junctions are connected in series in this manner until all junctions are connected and there is one free lead wire connected to a reference junction and one free lead wire connected to a sample junction. The free sample and reference lead wires will both be either positive or negative. The differential temperature between the sample and reference systems can be determined from the voltage across these wires. In the case of a thermopile sensor, the sensitivity of the sensor is equal to the product of the number of thermocouple junctions on the sample or reference side, the Seebeck coefficient of the thermocouple pair and the thermal resistance of the sensor. Thus, higher output sensors can be made by using a thermopile to measure the temperature difference.
The prior art includes a number of different methods for constructing thermopile DSC sensors. These include: deposition of the thermopile on an electrically insulating substrate using thin-film techniques, application of the thermopile to an electrically insulating substrate using thick-film techniques such as silk-screen printing, brazing metal thermocouple alloys to one another and to ceramic components, and joining protected electrically insulated thermocouples to a sensor structure comprising a thermal resistance. U.S. Pat. No. 5,033,866 to Kehl et al. and U.S. Pat. No. 5,288,147 to Schaefer et al. disclose thermopile DSC sensors fabricated using thick-film techniques. U.S. Patent Application 2008/0080591 to Tanaka et al. discloses thermopile DSC sensors fabricated by brazing metal thermocouple alloys to one another and to ceramic components. U.S. Patent Application 2011/0188534 to Nishimura et al. discloses thermopile DSC sensors where protected electrically insulated thermocouples are joined to a sensor structure comprising a thermal resistance.
However, each of these construction methods has certain disadvantages. For example, in thermopile sensors constructed by thin-film methods, the thermocouple materials are in the form of thin films that are deposited by evaporation of the materials. This generally limits the selection of material to pure metals, excluding the use of alloys. This restricts the choice of thermocouple materials to thermocouples that have generally low Seebeck coefficients. Therefore, sensors constructed using thin-film techniques tend to have low sensitivity. Given that the deposited films are very thin, the electrical impedance of the thermopile is quite high. This high impedance results in high electrical noise in the electronic circuitry that amplifies the differential temperature signal.
Thick film thermopile DSC sensors also have disadvantages. The thick film materials are a mixture of powdered thermocouple alloys, ceramics, glass frit, binders and organic solvents. They are applied to the substrate in liquid form, often by screen printing, dried and fired to form a solid coating on the substrate. By comparison with solid metal thermocouple alloys, their thermoelectric characteristics may vary considerably because the resultant mixture of powdered metals and binders may be inhomogeneous and may not conform to standards for the given thermocouple type. They also have much higher electrical resistivity than solid alloys and, like thin-film devices, also suffer from high impedance and the attendant amplification noise.
DSC sensors constructed by brazing thermocouple alloys and ceramic components avoid many of these problems but instead have unique problems resulting from the use of brazing. A wide selection of thermocouple alloys may be used and low sensor impedance can be achieved because solid metal thermocouple alloys are used. Brazing is a liquid phase joining process where the braze alloy melts at a lower temperature than the materials being joined, wets the surfaces of the base materials to form intimate contact and solidifies, joining them. Often, the liquid braze alloy dissolves the base materials forming other alloys. The presence of the braze alloy and any intermediate alloys that may form introduces additional thermoelectric materials into the thermopile potentially causing its output to differ from the standard for the thermocouple type. Thus, the output of the thermopile will not match the thermocouple standard, possibly introducing measurement errors.
Also, an important characteristic of a braze alloy for joining a particular alloy or combination of alloys is its ability to wet the base material. Good wetting is essential to forming reliable braze joints. Braze alloys that wet base materials well tend to flow along the surface of the base materials when they melt, making containment of the braze alloy difficult. Braze alloys that coat the surface of the thermocouple alloy may introduce additional thermoelectric elements into the thermopile, altering its output from the standard for the thermocouple type, possibly introducing measurement errors. The ceramic parts of the sensor that are brazed to the thermocouple junctions electrically insulate the thermocouple junctions from one another. If the braze alloy joining a thermocouple junction to a ceramic component flows across the surface of the ceramic, it may form a connection with an adjacent junction shorting the junctions, making the sensor inoperative.
Protected, electrically insulated thermocouples have one or more thermocouples that are surrounded by a ceramic electrical insulator and enclosed within a metal protection tube. When used in a thermopile DSC sensor, the protected thermocouples must be thermally connected to the sensor thermal resistance. In some DSCs, such as the DSC disclosed in U.S. Patent Application No. 2011/0188534, the thermocouple protection tubes may be brazed to the sensor thermal resistance. The ceramic electrical insulation between the thermocouple and the protection tube acts as a thermal insulator between the thermal resistance and the thermocouple. It reduces the sensitivity and speed of response of the thermocouple to sample heat flows that create the temperature differences across the thermal resistances. The thermocouple assemblies may have significant heat capacity which increases the heat capacity of the DSC sensor assembly, reducing its responsiveness and its ability to respond to rapid changes in sample heat flow. To keep the thermocouple heat capacity as low as possible, very small diameter protection tubes are employed, which in turn requires that the thermocouple wires be very fine. For that reason, the thermocouple has a relatively high electrical impedance. This tends to create high noise in the amplifier stages, because the sensor consequently has a high impedance.
Most heat flux DSCs employ a single differential temperature measurement and the simplified measurement method described above. It is well known that the simplified measurement method does not correctly measure the sample heat flow rate under many important experimental conditions. In particular, when a physical transformation occurs in the sample, the sample and reference heating rates are not the same. Consequently the measured heat flow rate may be significantly different from the actual sample heat flow rate. The simplified measurement method is based on the assumption that the DSC is perfectly symmetrical, i.e., the sample measuring system and the reference system are identical. As is well known, perfect symmetry is rarely achieved, such that the resulting heat flow rate measurement generally includes artifacts resulting from the asymmetry between the sample and reference measurement systems. An example is the DSC zero line when the instrument is operated without a sample or a reference. The heat flow rate should be very close to zero, but rarely is. The deviation from zero heat flow rate for an empty instrument is evidence that the instrument is not symmetric as assumed.